In conventional passive-type dynamic damper, an eigenvalue of the vibration absorber is uniquely determined because physical properties of the configuration elements are fixed. Accordingly, when a frequency of the disturbance vibration matches the eigenfrequency of the dynamic damper, a high damper effect is given. However, when the frequency of the disturbance vibration is slightly different from the eigenfrequency or when the frequency of the disturbance vibration varies as time passes, i.e., when the frequency is in an unstable state, the damper effect of the dynamic absorber cannot be fully given. Regarding this problem, a method of designing an optimal tuning and optimal damping to keep a damping effect over a certain frequency band is known. However, this cannot provide a damping effect that is lower than the response amplitude defined by the fixed point (unmoving point) on the frequency response curve.
On the other hand, as a counter part of the passive type of damping device, an active mass damper is known. In this method, an actuator for forcibly vibrating a mass is installed in the device in addition to the mass element. Accordingly, any arbitrary damping force can be generated theoretically, though any disturbance is applied. As a result, this method can provide a high damping effect irrespective of normal and abnormal states. However, there are known problems in that a device to directly apply outer energy becomes complicated, that it becomes necessary to design a control system to avoid unstableness, and that a cost is high. Further, there is a semi-active type of a control method which is an intermediate type between the passive type and the active type. In the semi-active type of damper, a physical property of one of configuration elements, which is originally fixed in the passive type system, is made variable with some means to have variability in the physical characteristic in the system. This enables the system control while the variation due to a disturbance is tracked to some extent. The semi-active type of control method has advantageous effects such as a control performance close to the active type of control method with reliability and stability inherently derived from the passive elements and provides a device at a cost which is lower than that of the active type of control.
Patent Document 1 disclosed a configuration in which an elastic modulus of the magnetic elastomer is changed by application of the magnetic field generated by an exciting coil by arranging an exciting coil at an outer circumferential area of the magnetic viscoelastic elastomer having elastic modulus which varies by application of the magnetic field.